1. Field of the Invention
This invention relates to a contact material for a vacuum valve and a method of manufacturing the same.
2. Description of the Related Art
The most important properties which contact material for vacuum valves is required to have are the three basic requirements of anti-welding property, voltage withstanding capability and current interrupting property. Further important requirements are to show low and stable rise in temperature and low and stable contact resistance. However, it is not possible to satisfy all these requirements by a single metal, as some of them are contradictory. Consequently, many of the contact materials that have been developed for practical use consist of combinations of two or more elements so as to complement their mutual deficiencies in performance, and to match specific applications such as large-current use or high voltage-withstanding ability. However, performance requirements have become increasingly severe and the present situation is such that these materials are unsatisfactory in some respects. A marked recent tendency is towards expansion of the use of these materials to capacitor circuits. Corresponding development and improvement of contact materials is an urgent task.
In order to cope with this, contact materials have previously been employed consisting of copper, as conductive constituent, combined with tungsten, molybdenum, tantalum or niobium, which are high melting point materials and in general provide excellent withstand-voltage capability.
Such Cu-W or the like contact materials can be applied in fields where a certain degree of withstand-voltage performance is required. However, they are subject to the problem of restriking in more severe high withstand-voltage regions and circuits in which inrush currents occur. The reason for this is insufficient adhesive strength between the grains of the arc-proof material and the conductive constituent, owing to insufficient wetting of the arc-proof material by the conductive constituent.
Specifically, restriking occurs, even though the electrodes are in open condition, because particles of arc-proof material get electrically charged and are discharged from the surface of the contacts, and because gas is emitted from pores produced in the interior of the contacts by insufficient wetting. Furthermore, when local welding takes place due to radio frequency currents etc. generated when the circuit is closed, since the interface between the aforementioned arc-proof material and conductive constituent is weak and local pores are present, transfer to the contact surface occurs when the electrodes are separated. This causes electric field concentrations etc., which may result in restriking. Such restriking may cause malfunctioning of the circuit system, resulting for example in cut-off of power. In particular, in capacitor circuits, a voltage of twice the ordinary circuit voltage is applied, so the problem of the withstand-voltage characteristic of the contacts, in particular, suppression of restriking has become prominent.
As described above, the reason for occurrence of restriking is insufficient strength of adhesion between the grains of arc-proof material and the conductive constituent, due to insufficient wetting of the arc-proof material with the conductive constituent. It is therefore vital to reduce the frequency of occurrence of restriking by increasing the interface strength and reducing internal pores.